Magnetoresistive sensors can include anisotropic magnetoresistive (AMR), giant magnetoresistive (GMR), tunnel magnetoresistive (TMR) and other technologies, referred to collectively as xMR technologies. XMR sensors can be used for a variety of applications, including magnetic field and current sensors, speed sensors, rotation sensors and angle sensors, among others.
Conventional AMR angle sensors are inherently limited to an angle uniqueness of 180 degrees due to the 90-degree uniqueness of the AMR effect with respect to a rotating external in-plane field. Thus, there are many applications for angle sensors with greater than 180-degree range that are instead fulfilled by angle sensors based on GMR or TMR technology. These technologies, however, suffer from considerable accuracy drift, especially at high magnetic fields in combination with high temperatures, which prevents GMR usage in applications with high accuracy requirements. Because AMR technology exhibits no magnetic field dependent accuracy drift, it is typically the preferred technology choice for very high accuracy applications.
Solutions exist that attempt to extend the range of AMR angle sensors to 360 degrees. For example, some increase the anisotropic field strength using an additional device, such as an integrated coil or hard magnetic bias layer. Such solutions often require that the external magnetic field be lower than the sensor anisotropy field strength, though, which limits the usually desired high magnetic fields. Other solutions apply an additional, or “auxiliary,” magnetic field, such as by integrated coils, which can be lower than the external magnetic field in order to allow the film magnetization to rotate. Drawbacks of these solutions, however, include increased power consumption and limited maximum measuring field, as well as increased measuring time due to the application of the temporary auxiliary field.
Therefore, a need remains for improved xMR sensors, including an AMR sensor having a 360-degree range.